This invention relates to a method of introducing impurity species into a semiconductor structure and more particularly to the introduction of impurity species from a thin film source deposited on a semiconductor structure employing a two step annealing treatment to bring about interdiffusion of elemental constituents making up the structure, which is known in the art as impurity induced disordering (IID).
It has been recognized that impurity diffusion into Group III-V compound semiconductors is an important step in the fabrication of optoelectronic devices. Recently, much attention has been given to the diffusion of Si into GaAs. Also, recently, considerable attention has been given to impurity induced disordering (IID) in GaAs/GaAlAs quantum well structures.
In particular, the diffusion of silicon in GaAs has been under study and investigation for many years. As an example, G. R. Antell in an article, "The Diffusion of Silicon in Gallium Arsenide", Solid-State Electronics, Vol. 8, pp. 943-946 (1965), discloses the diffusion of Si into GaAs carried out at high temperatures in a sealed quartz ampoule containing an overpressure of As to prevent the outdiffusion of As from the GaAs. The diffusivity and activation of Si in GaAs is proportional to the As overpressure and the Ga vacancy concentrations. Excess As pressure in a closed ampoule is required for successful diffusion. See, also, the more recent article on this subject of E. Omura et al, "Closed-Tube Diffusion of Silicon in GaAs From Sputtered Silicon Film", Electronic Letters, Vol. 22 (9), pp. 496-498 (Apr. 24, 1986).
More recently, the effects of encapsulation relative to Si implanted into GaAs have been studied to prevent the outdiffusion from GaAs and provide, in some cases, a source of Si for diffusion into GaAs. See the article of T. Onuma et al, "Study of Encapsulants for Annealing Si-Implanted GaAs", Journal of Electrochemical Society, Vol. 129 (4), pp. 837-840 (April, 1982). Diffusion of Si was enhanced by SiO.sub.2 encapsulation but was negligible with Si.sub.3 N.sub.4 encapsulation or when capless. The activation of the diffusion process is initiated at high anneal temperatures, such as 750.degree. C. and above. Onuma et al indicates that the SiO.sub.2 cap layer is permeable to Ga while the Si.sub.3 N.sub.4 cap layer is impermeable to Ga and As so that SiO.sub.2 permits the diffusion of Ga to provide for Ga vacancies in the GaAs and the substitution of Si. However, due to the deposition method employed, cracks developed in some of the samples when subjected to the subsequent high temperature annealing process. M. E. Greiner et al in the article, "Diffusion of Silicon in Gallium Arsenide Using Rapid Thermal Processing: Experiment And Model", Applied Physics Letters, Vol. 44 (8), pp. 750-752 (Apr. 15, 1984) examines Si diffusion from a thin elemental deposited source thereof using rapid thermal processing with several difficult encapsulants. The results show that diffusion was dependent on the type of encapsulant. The Si source layer and the encapsulants were deposited at relative low temperatures, i.e., below 450.degree. C., with subsequent annealing being accomplished at high temperatures of 850.degree. C.-1050.degree. C. for times from 3 seconds to 300 seconds providing diffusion depths to 0.2 .mu.m. High concentrations of Si diffused into GaAs resulted from a SiO.sub.2 capped thin Si source layer. In particular, a model proposed by Greiner et al explains that paired Si atoms can move substitutionally by exchanging sites with either Ga or As vacancies which explains the enhanced diffusion when using a SiO.sub.2 cap.
There have been recent investigations in the art of impurity diffusion studying the effect of rapid thermal annealing in comparison with standard furnace annealing for the compositional disordering of Si implanted GaAs/GaAlAs superlattices. The separate investigations of these two annealing approaches demonstrated that employing rapid thermal annealing (970.degree. C. for 10 seconds), the implantation damage in the sample could be eliminated without disordering of the superlattice but disordering of the superlattice would occur using furnace annealing (850.degree. C. for 30 minutes). See the article of J. Kobayashi et al, entitled "Effect of Rapid Thermal Annealing For the Compositional Disodering of Si-Implanted AlGaAs/GaAs Superlattices", Applied Physics Letters, Vol. 50 (9), pp. 519-521, Mar. 2, 1987. The conclusion reached was that the difference between disordering and not disordering in using these independent annealing processes was primarily the extent and magnitude of the Si diffusion into the superlattice.
K. L. Kavanagh et al in the articles, "Silicon Diffusion at Polycrystalline-Si/GaAs Interfaces", Applied Physics Letters, Vol. 47 (11), pp. 1208-1210 (Dec. 1, 1985) and "The Polycrystalline-Si Contact to GaAs", Journal of the Electrochemical Society, Vol. 133 (6), pp. 1176-1179 (June, 1986), reveals that, under proper conditions, the addition of As to the Si source layer revealed further enhanced diffusion, i.e., greater concentrations, of Si into the GaAs. These conditions called for depositing hydrogenated amorphous Si (a-Si:H) onto GaAs in a silane plasma at 450.degree. C. and subsequent annealing at temperatures between 600.degree. C.-1020.degree. C. The results showed that high level interdiffusion of Si atomic pairs with Ga and As vacancies occurs when As is initially added to the Si source layer. However, the surface area of films deposited onto GaAs continued to have a large number of randomly spaced bubbles, indicative of compressive stresses in the film, developed after the high temperature annealing process.
In copending application Ser. No. 117,593, filed concurrently with this application, a thin film bilayer composite source is deposited on a semiconductor structure that is stable at annealing temperatures required for the activation of the diffusion process so that pin holes, cracks, bubbles or other such irregularities do not appear in the deposited film or in the underlying semiconductor structure. In using a thin film deposited source for incorporating an impurity species or diffusant, such as Si, the level of impurity incorporation and the depth of impurity penetration into the crystal bulk increases with increasing diffusion temperature. However, it is sometimes desirable to vary the surface concentration of the impurity species independently of the total diffusion depth. Also, it is important in some applications to initially have a higher concentration of impurity species at the surface of the crystal bulk prior to thermal annealing. This capability would permit sharper transitions of ordered/disordered profile and better control over the performance of IID.
Thus, it is an object of this invention to provide a method of enhanced introduction of impurity species into a semiconductor structure from a deposited impurity source without incurring bubbles or cracks or other irregularities in the film or to be underlying semiconductor structure with improved control over the incorporation of the diffusant, the rate and depth of diffusion and the reproducibility of desired diffusion profiles, leading to improved device yields.